1. Field of the Invention
The present invention relates to a method for driving a cold cathode flat fluorescent lamp (CCFFL) and, more particularly, to a method for driving a CCFFL that enables the luminance uniformity of the CCFFL to be substantially constant while the CCFFL is adjusted for a desired luminance. Specifically, the method improves the luminance uniformity of the CCFFL when it is dim.
2. Description of the Related Art
The cold cathode flat fluorescent lamp (CCFFL) is used as a back-light source for an LCD display device or an LCD projector, or as a light source for a vehicle. The general construction of a CCFFL 100 is shown in FIG. 1, which is provided with a luminous surface 101, and a pair of electrodes 102 and 103. A conventional driving apparatus for the CCFFL 100 is composed of a power supply 104, a pulse generator 105, and an inverter driver 107, as shown in FIG. 2. In the conventional driving apparatus for the CCFFL 100, a conventional driving method for the CCFFL 100 comprises the steps of generating a pulse signal 106 by the pulse generator 105 of FIG. 2, applying the pulse signal 106 to the inverter driver 107, and generating a control signal by the inverter driver 107 to drive the CCFFL 100 to light up. In general, the control signal generated by the inverter driver 107 is a pulse signal or an alternating signal.
In addition, if it is necessary to adjust the luminance of the CCFFL 100, a pulse width PW of the pulse signal 106 of FIG. 3 is adjusted so as to alter the luminance of the CCFFL 100. Furthermore, a pulse period PT of the pulse signal 106 is set to be less than the time of visual persistence for human beings, so that the power consumption of the CCFFL 100 with the same luminance is reduced.
Thus, using the conventional driving method for the CCFFL 100, it is possible to alter the luminance of the CCFFL 100 and reduce the power consumption thereof by adjusting the pulse width PW and the pulse period PT of the pulse signal 106, respectively. However, due to the characteristic of the CCFFL 100, that is, when the pulse width PW is lowered to alter the luminance of the CCFFL 100, the luminance at electrodes 102 and 103 are brighter than the luminance at the central region of the luminous surface 101, such that a non-uniformity of luminance in the CCFFL 100 occurs. In addition, since the pulse width PW is lowered compared to the same pulse period PT, the effective pulse period becomes longer, that is, it is equal to, or longer than, the time of visual persistence for human beings, thereby unstable luminance of the CCFFL 100 occurs.
Therefore, it is difficult to have a desired luminance uniformity while the CCFFL is adjusted for a desired reduced luminance using the conventional driving method for the CCFFL.
It is an object of the invention to provide a method for driving the cold cathode flat fluorescent lamp (CCFFL) for enabling the luminance uniformity of the CCFFL to be substantially constant while the CCFFL is adjusted for a desired luminance.
It is another object of the invention to provide a method for driving the CCFFL suitable for improving the luminance uniformity of the CCFFL when it is dim.
It is still another object of the invention to provide a method for driving the CCFFL suitable for reducing the power consumption thereof and preventing the luminance of the CCFFL from being unstable.
According to an aspect of the invention, there is provided the method for driving the CCFFL comprising the following steps: generating a pulse-combined signal, wherein the pulse-combined signal has a plurality of pulse signals, and each of the plurality of pulse signals has a pulse width and a pulse period; applying the pulse-combined signal to the inverter driver circuit, wherein the inverter driver circuit has a pair of output terminals, which are electrically connected to a pair of input terminals on the CCFFL so as to input a control signal to the CCFFL to light it up; adjusting the pulse width and the pulse period so that the CCFFL is at a first luminance while a first luminance uniformity thereof is maximum; setting the pulse width and the pulse period such that the CCFFL is at the first luminance and have the first luminance uniformity; and generating the pulse-combined signal having a turn-off time that allows the CCFFL to be at a second luminance and to have a second luminance uniformity; wherein the second luminance uniformity is substantially equal to the first luminance uniformity.
It is preferred that the step of generating the pulse-combined signal having the turn-off time comprises using a sinusoid-wave signal, a square-wave signal, or a triangle-shaped wave signal, and performing a modulating operation.
It is advantageous that the method for driving the CCFFL further comprises a step of generating the pulse-combined signal having a turn-on time that allows the CCFFL to be at a third luminance and to have a third luminance uniformity; wherein the third luminance uniformity is substantially equal to the first luminance uniformity.
It is preferred that the step of generating the pulse-combined signal having the turn-on time comprises using a sinusoid-wave signal, a square-wave signal, or a triangle-shaped wave signal, and performing a modulating operation.
It is preferred that the third luminance is the desired luminance of the CCFFL.
It is preferred that the third luminance is the first luminance of the CCFFL.
It is preferred that the first luminance is the maximum luminance of the CCFFL.
It is preferred that the second luminance is the desired luminance of the CCFFL.
Other objects and advantages of the present invention will become apparent from the detailed description to follow taken in conjunction with the appended claim.